1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer.
2. Description of Related Art
A conventional image forming apparatus such as an electrophotographic printer generally executes a series of operations in image forming processes such as charging, exposing, developing, transferring and fixing.
According to the image forming processes, a photoconductive insulation layer on a surface of a photosensitive drum is uniformly charged in the charging process. Subsequently, the photoconductive insulation layer on the surface of the photosensitive drum is exposed, so that an electric charge on the exposed area is extinguished, thereby forming an electrostatic latent image thereon in the exposing process. In the development process, the electrostatic latent image is developed by adhesion of toner having a color agent thereto by using a development roller, a toner conveyance roller and the like. The toner image is transferred onto a transfer medium such as a recording sheet in the transfer process. The toner image on the transfer member is fixed by heat, pressure or an appropriate fixing manner in the fixing process.
Such an image forming apparatus of prior art needs to precisely control an amount of the toner to be adhered to the transfer medium so as to reproduce the image correctly. The toner amount is determined by controlling a process condition. For example, density of a patch pattern or the like formed on a sheet conveyance belt such as a transfer belt is frequently measured to control the process condition based on the measured data.
Japanese Un-examined Patent Application Publication No. 2004-029681 discloses a method of controlling a process condition that a printer controller changes a potential difference between an electric potential to be applied to a development roller serving as a developer carrier and an electric potential to be applied to a toner conveyance roller serving as a developer supply carrier based on a dot number per A4-sized sheet of image data before the development process.
However, such a prior art method of controlling the process condition causes a blurring image due to inadequate image density. For example, when an image with a high toner density such as an image density of 100 percent (e.g., solid image) is printed, an adequate toner amount is adhered to a leading end of the image on the recording sheet during the beginning of the printing. However, a supply shortage of the toner to the development roller occurs during the end of the printing to print a tailing end of the image. Consequently, the printed result cannot obtain an adequate image density, and there raises a problem that so called phenomenon of “blur” may occur. Since the printer controller determines the voltage to be applied to the development roller and the toner conveyance roller before the development process, the determined voltage cannot be changed during the development process, thereby causing the blurring image.